1. Field of the Invention
This invention relates to a metal salt composition, in particular for the removal of impurities from combustion gases. The invention further relates to a process for the removal of impurities from combustion gases using such a metal salt composition.
2. Description of the Prior Art
Greater importance today is being placed on the manufacture and use of combustion gases. Combustion gases include the gaseous products from complete or partial combustion, specifically including, e.g., synthesis gas, pyrolysis gas and flue gases. The synthesis gas may have been formed by gasification of, for instance, a heavy mineral oil fraction or coal.
The impurities occurring in this type of gases are partly of a solid nature, called fly ash particles, and partly of a gaseous nature, e.g., H.sub.2 S, COS and organic sulfur compounds. It is essential that these impurities be removed in view of environmental and technical considerations.
One method to remove these impurities at ambient temperature and pressure is by use of a tetrahydrothiophene dioxide/alkanolamine mixture. But if the gases are to be used as feed for a gas turbine, as will be the case in the prospective combinations of a coal gasification plant with an electric power station ("combined cycle power plants"), it is not very efficient to carry out this gas purification at a low temperature. This is because the efficiency of the combination will be higher when the purification is carried out at higher temperatures, since in this way the perceptible heat will be conserved better. As the maximum inlet temperature of a gas turbine--before combustion--is about 550.degree. to 600.degree. C., the gas purification should preferably be carried out at that temperature.
Only a small number of liquids are suitable for use as absorbent liquids at that temperature, and those liquids almost invariably consist of molten salts. However, the salt or mixture of salts used should meet certain requirements, which will restrict the range of choice:
(1) good capacity for the selective removal of H.sub.2 S, COS and organic sulfur compounds, PA1 (2) stability at the prevailing temperature, PA1 (3) very low vapor pressure, PA1 (4) low corrosivity, PA1 (5) easy regenerability, i.e., reconversion of the sulfides formed into the original metal salt composition, PA1 (6) inertness with regard to the combustion gas, and PA1 (7) inertness with regard to the inorganic solid impurities, so that the latter can easily be removed by hot filtration.
A well-known composition which meets some of these requirements is the ternary mixture of potassium carbonate, lithium carbonate and sodium carbonate, which is fluid at the prevailing temperature, along with some calcium carbonate dissolved therein as an active component. Such mixtures and their use in purifying gas streams are obtained in U.S. Pat. Nos. 3,919,390 and 4,086,323. The ternary mixture is fluid because the three components can constitute an eutectic mixture, the melting point of the pure eutectic being 397.degree. C. The absorbancy of this metal salt composition is high at a temperature of, say, 700.degree. C., but regeneration--by steam and carbon dioxide being passed through--is slow and should preferably be carried out at the lowest possible temperature in order that the H.sub.2 S gas formed may be sufficiently concentrated for processing in a Claus plant. Operation at a low temperature, i.e., as little as possible above the eutectic temperature, entails the risk of sudden solidification of the melt owing to solid impurities (fly ash) and products of corrosion. It has further been found that only a limited quantity of calcium carbonate can be dissolved in the mixture of lithium carbonate, sodium carbonate and potassium carbonate. Moreover, the maximum quantity of sulfur that can be removed by the calcium carbonate is rather limited, in other words, at the absorption temperature of about 550.degree. C. aimed at, the equilibrium EQU CaCO.sub.3 +H.sub.2 S.fwdarw.CaS+H.sub.2 O+CO.sub.2
lies not entirely to the right.